The production of nitrogen by pressure swing adsorption is traditionally accomplished using a form of activated carbon known as a carbon molecular sieve adsorbent. Because a carbon molecular sieve can differentiate between the molecular sizes of nitrogen and oxygen, thereby adsorbing oxygen more readily than nitrogen, the pressure swing adsorption process can provide a high purity nitrogen product.
The key properties of a carbon molecular sieve adsorbent are adsorption capacity, gas uptake rate and kinetic selectivity. In general, there is a compromise between the kinetic selectivity of a carbon molecular sieve adsorbent and the gas uptake rate, where raising the selectivity invariably results in slower uptake rates. If a carbon molecular sieve adsorbent is made highly selective, the gas uptake rates become very slow, resulting in limitation in productivity of the pressure swing adsorption process. Because of this, the selectivity of a carbon molecular sieve adsorbent can only be enhanced to some less than optimal value.
On the other hand, increasing the capacity of the carbon molecular sieve adsorbent can improve the performance of the pressure swing adsorption process without any detrimental side effects. By creating a carbon molecular sieve with improved capacity without sacrificing its ability to separate nitrogen from oxygen, a pressure swing adsorption process could be utilized with smaller carbon molecular sieve beds, significantly driving down the price of a nitrogen pressure swing adsorption process by reducing the required size. Prior to the advent of carbon molecular sieves, activated carbons were used for various adsorptive processes. The creation of activated carbons begins with the carbonaceous precursor, such as coconut shells, wood, fruit pits, nut shells, coal, peat or bone. The use of metals and metal compounds to enhance various capabilities of activated carbon have also been attempted.
U.S. Pat. No. 1,551,074 discloses a process for impregnating ground peat with iron chloride which is thereafter dried and calcined to produce an activated carbon purportedly with good adsorptive capacity and great rigidity.
U.S. Pat. No. 1,559,054 discloses a process for improving the decolorizing, deodorizing and adsorptive properties of activated carbon by including nickel, palladium, platinum, copper, zinc, osmium, thorium or aluminum in the metallic form in crude organic material prior to burning to form an activated carbon.
U.S. Pat. No. 1,845,815, discloses the treatment of carbon, such as coal, with sodium carbonate before being heated slowly in the presence of air to approximately 240.degree. C. with subsequent removal of the sodium carbonate by washing with water or dilute acid.
U.S. Pat. No. 2,025,367 discloses a process for treating carbon such as peat, in a carbonizing process followed by application of zinc chloride and heating to 350.degree. C. The resulting material is then rinsed with water and dilute acid to remove the chlorides.
U.S. Pat. No. 3,355,317 discloses an adsorbent of carbon for cigarette filters with the impregnation of metals complexed with ammonia such as cobalt, copper, zinc, iron, molybdenum and silver, followed by heating to produce a metal oxide in the carbon filter material. Metal chlorides are not desirable as metal impregnants. Temperatures up to 500.degree. C. are used to heat treat the composite, potentially in an inert atmosphere.
U.S. Pat. No. 3,886,093 describes a process for making activated carbons using lignin impregnated with various transition metal salts, which are then carbonized to produce the activated carbon with active metal sites. The carbonization can be performed at a temperature in the range of 700.degree. to 1800.degree. F.
U.S. Pat. No. 4,082,694 discloses a process for making activated carbons by contact with potassium hydroxide and pre-calcining in a temperature in the range of 600.degree. to 900.degree. F. followed by dehydration and calcining at a temperature in the range of 1300.degree. to 1800.degree. F. to provide an activated carbon useful as an adsorbent for vapors and other adsorptive properties.
U.S. Pat. No. 5,063,196 discloses an activated carbon made by the impregnation with copper, zinc and silver in several steps and heated at a temperature up to 180.degree. C. The resulting product has utility as an adsorbent in gas masks.
U.S. Pat. No. 5,071,820 discloses a process for making a microporous carbon in a two-stage heat treatment process. Coal or a polymer is first heated up to 200.degree. C. in air and then is pyrolyzed in nitrogen at a temperature up to 850.degree. C. Metal impregnants are not identified.
British Patent 1,375,900 discloses a method for making a carbon molecular sieve wherein metal compounds are either impregnated in a carbon precursor or a subsequent formation of carbon molecular sieve to produce adsorbents for various utilities based upon the particular metal impregnated. Platinum, iron and copper are exemplified within the patent.
British patent 2,187,725 discloses the preparation of an adsorbent for a gas mask which uses various metal impregnants on carbons or activated carbons including cobalt, chromium, nickel, zinc and Group VIII transition metal salts. No significant heating is performed after the impregnation of the metal salts on the carbon substrate or support.
The article "Catalytic and Surface Property of Activated Carbon Impregnated with Cu.sup.2+, Ni.sup.2+ and Cr.sup.3+ " by M. M. Selim, et al., appearing in Afinidad, November/December 1990, pages 408 through 410, discloses impregnated and activated carbon with copper, nickel or chromium salts and post impregnation activation at a temperature up to 500.degree. C. in an inert helium atmosphere. The materials are used for catalysts rather than adsorbents.
In an article "Gasification of Active Carbons of Different Texture Impregnated with Nickel, Cobalt and Iron" by J. L. Figueiredo, et al., appearing in Carbon, Volume 25 No. 5, page 703 through 708 (1987), activated carbons were impregnated with nickel, cobalt and iron prior to gasification wherein the metals constitute catalysts for improved gasification of the activated carbons.
The prior art has impregnated various metals on various carbon and activated carbon precursors, typically to provide catalytic effect and chemi-sorption properties to the resulting carbons and activated carbons. However, these uses of metals, which are basically supported on the carbon substrate, are for their direct catalytic or chemi-sorption properties and were not implemented into the carbons for their effect on the pore geometry of the carbons themselves. Therefore, the prior art has not provided a solution to the problem of attaining high performance carbon adsorbents, which have increased capacity through enhanced microporosity. This problem is overcome by the present invention as will be set forth below wherein a high capacity, fast carbon adsorbent, and preferably a carbon molecular sieve, having enhanced adsorbent properties is prepared by unique process with metal salt impregnations during the formation of microporosity in the synthesis of carbon molecular sieves.